Chapter 7: Clinical Treatment Planning in External Photon Beam - - PDF document

1 IAEA

International Atomic Energy Agency

Objective: To familiarize the student with a variety of modern photon beam radiotherapy techniques to achieve a uniform dose distribution inside the target volume and a dose as low as possible in the healthy tissues surrounding the target.

Chapter 7: Clinical Treatment Planning in External Photon Beam Radiotherapy

Set of 232 slides based on the chapter authored by

• W. Parker, H. Patrocinio
• f the IAEA publication:

Review of Radiation Oncology Physics: A Handbook for Teachers and Students

Slide set prepared in 2006 by G.H. Hartmann (Heidelberg, DKFZ) Comments to S. Vatnitsky: dosimetry@iaea.org

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.Slide 1

7.1 Introduction 7.2 Volume Definition 7.3 Dose Specification 7.4 Patient Data Acquisition and Simulation 7.5 Clinical Considerations for Photon Beams 7.6 Treatment Plan Evaluation 7.7 Treatment Time and Monitor Unit Calculations

CHAPTER 7. TABLE OF CONTENTS

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7.1 INTRODUCTION General considerations for photon beams

Almost a dogma in external beam radiotherapy:

Successful radiotherapy requires a uniform dose distribution within the target (tumor). External photon beam radiotherapy is usually carried out with multiple radiation beams in order to achieve a uniform dose distribution inside the target volume and a dose as low as possible in healthy tissues surrounding the target.

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Criteria of a uniform dose distribution within the target

Recommendations regarding dose uniformity, prescribing,

recording, and reporting photon beam therapy are set forth by the International Commission on Radiation Units and Measurements (ICRU).

The ICRU report 50 recommends a target dose uniformity

within +7% and –5% relative to the dose delivered to a well defined prescription point within the target. 7.1 INTRODUCTION

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beam energies

and

field sizes

To achieve this goal, modern beam radiotherapy is carried out with a variety of: 7.1 INTRODUCTION

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Beam energies used:

• superficial

(30 kV to 80 kV)

• orthovoltage

(100 kV to 300 kV)

• megavoltage or

supervoltage energies (Co-60 to 25 MV) 7.1 INTRODUCTION

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Field sizes range from:

.

small circular fields used in radiosurgery standard rectangular and irregular fields very large fields used for total body irradiations 7.1 INTRODUCTION

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Methods of Patient setup:

Photon beam radiotherapy is carried out under two setup

conventions constant Source-Surface Distance (SSD technique) isocentric setup with a constant Source-Axis Distance (SAD technique). 7.1 INTRODUCTION

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SSD technique

The distance from the source to the surface of the patient

is kept constant for all beams. 7.1 INTRODUCTION

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SAD technique

The center of the target volume is placed at the machine

isocenter, i.e. the distance to the target point is kept constant for all beams. 7.1 INTRODUCTION

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Note: In contrast to SSD technique, the SAD technique requires no adjustment of the patient setup when turning the gantry to the next field. 7.1 INTRODUCTION

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.2 Slide 1

7.2 VOLUME DEFINITION

The process of determining the volume for the treatment

• f a malignant disease consists of several distinct steps.

In this process, different volumes may be defined, e.g.

due to:

• varying concentrations of malignant cells
• probable changes in the spatial relationship between

volume and beam during therapy

• movement of patient
• possible inaccuracies in the treatment setup.

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The ICRU 50 and 62 Reports define and describe several target and critical structure volumes that:

• aid in the treatment

planning process

• provide a basis for

comparison of treat- ment outcomes. 7.2 VOLUME DEFINITION

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The following slides describe these "ICRU volumes" that have been defined as principal volumes related to three- dimensional treatment planning. 7.2 VOLUME DEFINITION

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7.2 VOLUME DEFINITION

7.2.1 Gross Tumor Volume (GTV)

The Gross Tumor Volume (GTV) is the gross palpable or

visible/demonstrable extent and location of malignant growth.

The GTV is usually based on information obtained from a

combination of imaging modalities (CT, MRI, ultrasound, etc.), diagnostic modalities (pathology and histological reports, etc.) and clinical examination.

GTV

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7.2 VOLUME DEFINITION

7.2.2 Clinical Target Volume (CTV)

The Clinical Target Volume (CTV) is the tissue volume

that contains a demonstrable GTV and/or sub-clinical microscopic malignant disease, which has to be eliminated.

This volume thus has to be treated adequately in order to

achieve the aim of therapy, cure or palliation.

GTV CTV

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7.2 VOLUME DEFINITION

7.2.2 Clinical Target Volume (CTV)

The CTV often includes the area directly surrounding

the GTV that may contain microscopic disease and other areas considered to be at risk and require treatment. Example: positive lymph nodes.

The CTV is an anatomical-clinical volume. It is usually determined by the radiation oncologist, often

after other relevant specialists such as pathologists or radiologists have been consulted.

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7.2 VOLUME DEFINITION

7.2.2 Clinical Target Volume (CTV)

The CTV is usually stated as a fixed or variable margin

around the GTV. Example: CTV = GTV + 1 cm margin

In some cases the CTV is the same as the GTV.

Example: prostate boost to the gland only

There can be several non-contiguous CTVs that may

require different total doses to achieve treatment goals.

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7.2 VOLUME DEFINITION

7.2.3 Internal Target Volume (ITV)

General consideration on margins:

Margins are most important for clinical radiotherapy.

They depend on:

• organ motion

internal margins

• patient set-up and beam alignment

external margins

Margins can be non-uniform but should be three

dimensional.

A reasonable way of thinking would be: “Choose

margins so that the target is in the treated field at least 95% of the time.”

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7.2 VOLUME DEFINITION

7.2.3 Internal Target Volume (ITV)

The Internal Target Volume (ITV) consists of the CTV

plus an internal margin.

The internal margin is designed to take into account the

variations in the size and position of the CTV relative to the patient’s reference frame (usually defined by the bony anatomy), i.e., variations due to organ motions such as breathing, bladder or rectal contents, etc.

CTV ITV CTV

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7.2 VOLUME DEFINITION

7.2.4 Planning Target Volume (PTV)

In contrast to the CTV, the Planning Target Volume

(PTV) is a geometrical concept.

It is defined to select appropriate beam arrangements,

taking into consideration the net effect of all possible geometrical variations, in order to ensure that the prescribed dose is actually absorbed in the CTV.

CTV ITV PTV

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7.2 VOLUME DEFINITION

7.2.4 Planning Target Volume (PTV)

The PTV includes the internal target margin and an

additional margin for:

• set-up uncertainties
• machine tolerances
• and intra-treatment

variations.

The PTV is linked to the

reference frame of the treatment machine (IEC 1217: "Fixed System").

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7.2 VOLUME DEFINITION

7.2.4 Planning Target Volume (PTV)

The PTV is often described as the CTV plus a fixed or

variable margin. Example: PTV = CTV + 1 cm

Usually a single PTV is used to encompass one or several

CTVs to be targeted by a group of fields.

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7.2 VOLUME DEFINITION

7.2.4 Planning Target Volume (PTV)

The PTV depends on the precision of such tools such as:

• immobilization devices
• lasers

The PTV does NOT include a margin for dosimetric

characteristics of the radiation beam as these will require an additional margin during treatment planning and shielding design. Examples not included:

• penumbral areas
• build-up region

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7.2 VOLUME DEFINITION

7.2.5 Organ at Risk (OAR)

Organ At Risk is an organ whose sensitivity to radiation

is such that the dose received from a treatment plan may be significant compared to its tolerance, possibly requiring a change in the beam arrangement or a change in the dose.

CTV ITV PTV OAR

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7.2 VOLUME DEFINITION

7.2.5 Organ at Risk (OAR)

Specific attention should be paid to organs that, although

not immediately adjacent to the CTV, have a very low tolerance dose. Example for such OARs:

• eye lens during naso-pharyngeal or brain tumor

treatments

Organs with a radiation tolerance that depends on the

fractionation scheme should be outlined completely to prevent biasing during treatment plan evaluation.

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7.3 DOSE SPECIFICATION

The complete prescription of radiation treatment must

include:

• a definition of the aim of therapy
• the volumes to be considered
• a prescription of dose and fractionation.

Only detailed information regarding total dose, fractional

dose and total elapsed treatment days allows for proper comparison of outcome results.

Different concepts have been developed for this

requirement.

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When the dose to a given volume is prescribed, the

corresponding delivered dose should be as homogeneous as possible.

Due to technical reasons, some heterogeneity has to be

accepted. Example:

PTV = dotted area frequency dose-area histogram for the PTV

7.3 DOSE SPECIFICATION

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The ICRU report 50 recommends a target dose uniformity

within +7% and –5% relative to the dose delivered to a well defined prescription point within the target.

Since some dose heterogeneity is always present, a

method to describe this dose heterogeneity within the defined volumes is required.

ICRU Report 50 is suggesting several methods for the

representation of a spatial dose distribution. 7.3 DOSE SPECIFICATION

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Parameters to characterize the dose distribution within a

volume and to specify the dose are:

• Minimum target dose
• Maximum target dose
• Mean target dose
• A reference dose at a representative point within the

volume

The ICRU has given recommendations for the selection of a

representative point (the so-called ICRU reference point). 7.3 DOSE SPECIFICATION

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The ICRU reference dose point is located at a point

chosen to represent the delivered dose using the following criteria:

• The point should be located in a region where the

dose can be calculated accurately (i.e., no build-up or steep gradients).

• The point should be in the central part of the PTV.
• For multiple fields, the isocenter (or beam intersection

point) is recommended as the ICRU reference point. 7.3 DOSE SPECIFICATION

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Example for a 3 field prostate boost treatment with an isocentric technique The ICRU (reference) point is located at the isocenter ICRU reference point for multiple fields 7.3 DOSE SPECIFICATION

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Specific recommendations are made with regard to the

position of the ICRU (reference) point for particular beam combinations:

• For single beam:

the point on central axis at the center of the target volume.

• For parallel-opposed equally weighted beams:

the point on the central axis midway between the beam entrance points.

• For parallel-opposed unequally weighted beams:

the point on the central axis at the centre of the target volume.

• For other combinations of intersecting beams:

the point at the intersection of the central axes (insofar as there is no dose gradient at this point).

7.3 DOSE SPECIFICATION

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7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.1 Need for patient data

Within the simulation process of the entire treatment using

the computerized treatment planning system, the patient anatomy and tumor targets can be represented as three- dimensional models. Example:

• CTV: mediastinum (violette)
• OAR:
• both lungs (yellow)
• spinal cord (green)

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Patient data acquisition to create the patient model is the

initial part of this simulation process.

The type of gathered data varies greatly depending on the

type of treatment plan to be generated. Examples:

• manual calculation of parallel-opposed beams

requires less effort

• complex 3D treatment plan with image fusion

requires large effort 7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.1 Need for patient data

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General considerations on patient data acquisition:

Patient dimensions are always required for treatment

time or monitor unit calculations, whether read with a caliper, from CT slices or by other means.

Type of dose evaluation also dictates the amount of

patient data required (e.g., DVHs require more patient information than point dose calculation of organ dose).

Landmarks such as bony or fiducial marks are required to

match positions in the treatment plan with positions on the patient. 7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.1 Need for patient data

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7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.2 Nature of patient data

The patient information required for treatment planning

varies from rudimentary to very complex data acquisition:

• distances read on the skin
• manual determination of contours
• acquisition of CT information over a large

volume

• image fusion using various imaging modalities

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The patient information required for treatment planning in

particular depends on which system is used: three-dimensional system two-dimensional system 7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.2 Nature of patient data

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2D treatment planning

A single patient contour, acquired using lead wire or

plaster strips, is transcribed onto a sheet of graph paper, with reference points identified.

Simulation radiographs are taken for comparison with port

films during treatment.

For irregular field calculations, points of interest can be

identified on a simulation radiograph, and SSDs and depths of interest can be determined at simulation.

Organs at risk can be identified and their depths

determined on simulator radiographs. 7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.2 Nature of patient data

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3D treatment planning

CT dataset of the region to be treated is required with a

suitable slice spacing (typically 0.5 - 1 cm for thorax, 0.5 cm for pelvis, 0.3 cm for head and neck).

An external contour (representative of the skin or

immobilization mask) must be drawn on every CT slice used for treatment planning.

Tumor and target volumes are usually drawn on CT

slices.

Organs at risk and other structures should be drawn in

their entirety, if dose-volume histograms are to be calculated. 7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.2 Nature of patient data

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Contours for different volumes have been drawn on this CT slice for a prostate treatment plan:

• GTV
• CTV
• PTV
• organs at risk

(bladder and rectum). 7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.2 Nature of patient data

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3D treatment planning (cont.)

MRI or other studies (PET) are required for image fusion. With many treatment planning systems, the user can

choose:

• to ignore inhomogeneities (often referred to as

heterogeneities)

• to perform bulk corrections on outlined organs
• to or use the CT data itself (with an appropriate

conversion to electron density) for point-to-point correction. 7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.2 Nature of patient data

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3D treatment planning (cont.)

CT images can be used to produce digitally reconstructed

radiographs (DRRs)

DRRs are used for comparison with portal films or beam’s

eye view to verify patient set up and beam arrangement A digitally reconstructed radiograph with super- imposed beam’s eye view for an irregular field 7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.2 Nature of patient data

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7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.3 Treatment simulation

Patient simulation was initially developed to ensure that

the beams used for treatment were correctly chosen and properly aimed at the intended target. Example: The double exposure technique The film is irradiated with the treatment field first, then the collimators are opened to a wider setting and a second exposure is given to the film.

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Presently, treatment simulation has a more expanded role

in the treatment of patients consisting of:

• Determination of patient treatment position
• Identification of the target volumes and OARs
• Determination and verification of treatment field

geometry

• Generation of simulation radiographs for each

treatment beam for comparison with treatment port films

• Acquisition of patient data for treatment planning.

7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.3 Treatment simulation

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.4.3 Slide 3

The comparison of simple simulation with portal image (MV) and conventional simulation with diagnostic radiography (kV) of the same anatomical site (prostate) demonstrates the higher quality of information on anatomical structures. Reference simulator film (kV) Check portal film (MV) 7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.3 Treatment simulation

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It is neither efficient nor practical to perform simulations

with portal imaging on treatment units.

• There is always heavy demand for the use of

treatment units for actual patient treatment

• Using them for simulation is therefore considered an

inefficient use of resources.

• These machines operate in the megavoltage range of

energies and therefore do not provide adequate quality radiographs for a proper treatment simulation. poor image quality! 7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.3 Treatment simulation

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Reasons for the poor quality of port films:

• Most photon interactions with biological material in the

megavoltage energy range are Compton interactions that produce scattered photons that reduce contrast and blur the image.

• The large size of the radiation source (either focal spot for a

linear accelerator or the diameter of radioactive source in an isotope unit) increases the detrimental effects of beam penumbra

• n the image quality.
• Patient motion during the relatively long exposures required and

the limitations on radiographic technique also contribute to poor image quality.

7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.3 Treatment simulation

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Therefore, dedicated equipment – fluoroscopic simulator - has been developed and was widely used for radiotherapy simulation. 7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.3 Treatment simulation

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Modern simulation systems are based on computed tomography (CT) or magnetic resonance (MR) imagers and are referred to as CT- simulators or MR- simulators. A dedicated radiotherapy CT simulator 7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.3 Treatment simulation

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7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.4 Patient treatment position and immobilization devices

Patients may require an external immobilization device for their treatment, depending on:

the patient treatment position, or the precision required for beam delivery.

Example: The precision required in radiosurgery

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.4.4 Slide 2

Immobilization devices have two fundamental roles:

To immobilize the patient during treatment; To provide a reliable means of reproducing the patient

position from treatment planning and simulation to treatment, and from one treatment to another. 7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.4 Patient treatment position and immobilization devices

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.4.4 Slide 3

The immobilization means include masking tape, velcro

belts, or elastic bands, or even a sharp fixation system attached to the bone. 7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.4 Patient treatment position and immobilization devices

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The simplest immobilization device used in radiotherapy is

the head rest, shaped to fit snugly under the patient’s head and neck area, allowing the patient to lie comfortably on the treatment couch. Headrests used for patient positioning and immobilization in external beam radiotherapy 7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.4 Patient treatment position and immobilization devices

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Other immobilization accessories:

Patients to be treated in the head and neck or brain areas

are usually immobilized with a plastic mask which, when heated, can be moulded to the patient’s contour.

The mask is affixed directly

• nto the treatment couch
• r to a plastic plate that lies

under the patient thereby preventing movement. 7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.4 Patient treatment position and immobilization devices

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For extra-cranial treatments (such as to the thoracic or pelvic area), a variety of immobilization devices are available. Vacuum-based devices are popular because of their re-usability. A pillow filled with tiny styrofoam balls is placed around the treatment area, a vacuum pump evacuates the pillow leaving the patient’s form as an imprint in the pillow. 7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.4 Patient treatment position and immobilization devices

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.4.4 Slide 7

Another system, similar in concept, uses a chemical reaction between two reagents to form a rigid mould of the patient. 7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.4 Patient treatment position and immobilization devices

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Another system uses the mask method adopted to the body. 7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.4 Patient treatment position and immobilization devices

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Special techniques, such as stereotactic radiosurgery,

require such high precision that conventional immobilization techniques are inadequate.

In radiosurgery, a stereotactic

frame is attached to the patient’s skull by means of screws and is used for target localization, patient setup, and patient immobilization during the entire treatment procedure. 7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.4 Patient treatment position and immobilization devices

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7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.5 Patient data requirements

For simple hand calculations of the dose along the central

axis of the beam and the beam-on time or linac monitor units, the source-surface distance along the central ray

• nly is required.

Examples:

• treatment with a direct field;
• parallel and opposed fields.

Requirement: a flat beam incidence.

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If simple algorithms, such as Clarkson integration, are

used to determine the dosimetric effects of having blocks in the fields or to calculate the dose to off-axis points, their coordinates and source to surface distance must be measured. The Clarkson integration method (for details see chapter 6) 7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.5 Patient data requirements

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.4.5 Slide 3

For simple computerized 2D treatment planning, the

patient’s shape is represented by a single transverse skin contour through the central axis of the beams.

This contour may be acquired using lead wire or plaster

cast at the time of simulation. 7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.5 Patient data requirements

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The patient data requirements for modern

3D treatment planning systems are more elaborate than those for 2D treatment planning.

The nature and complexity of data required limits the use

• f manual contour acquisition.

Transverse CT scans contain all information required

for complex treatment planning and form the basis of CT- simulation in modern radiotherapy treatment. 7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.5 Patient data requirements

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The external shape of the patient must be outlined for all

areas where the beams enter and exit (for contour corrections) and in the adjacent areas (to account for scattered radiation).

Targets and internal structures must be outlined in

• rder to determine their shape and volume for dose

calculation.

Electron densities for each volume element in the dose

calculation matrix must be determined if a correction for heterogeneities is to be applied. The patient data requirements for 3D treatment planning include the following: 7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.5 Patient data requirements

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7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.6 Conventional treatment simulation

A fluoroscopic simulator consists of a gantry and couch

arrangement similar to that on a isocentric megavoltage treatment unit.

The radiation source

is a diagnostic quality x-ray tube rather than a high-energy linac

• r a cobalt source.

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Modern simulators provide the ability to mimic most

treatment geometries attainable on megavoltage treatment units, and to visualize the resulting treatment fields on radiographs or under fluoroscopic examination of the patient. Adjustable bars made

• f tungsten can mimic

the planned field size superimposed to the anatomical structures. 7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.6 Conventional treatment simulation

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The photons produced by the x-ray tube are in the

kilovoltage range and are preferentially attenuated by higher Z materials such as bone through photoelectric interactions.

The result is a high quality diagnostic radiograph with

limited soft-tissue contrast, but with excellent visualization

• f bony landmarks and high Z contrast agents.

A fluoroscopic imaging system may also be included and

would be used from a remote console to view patient anatomy and to modify beam placement in real time. 7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.6 Conventional treatment simulation

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For the vast majority of sites, the disease is not visible on

the simulator radiographs

Therefore the block positions can be determined only with

respect to anatomical landmarks visible on the radiographs (usually bony structures or lead wire clinically placed on the surface of the patient). 7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.6 Conventional treatment simulation

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.4.6 Slide 5

Determination of treatment beam geometry

Typically, the patient is placed on the simulator couch,

and the final treatment position of the patient is verified using the fluoroscopic capabilities of the simulator (e.g., patient is straight on the table, etc.).

The position of the treatment isocenter, beam geometry

(i.e., gantry, couch angles, etc.) and field limits are determined with respect to the anatomical landmarks visible under fluoroscopic conditions. 7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.6 Conventional treatment simulation

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.4.6 Slide 6

Determination of treatment beam geometry

Once the final treatment

geometry has been established, radiographs are taken as a matter of record, and are also used to determine shielding requirements for the treatment.

Shielding can be drawn

directly on the films, which may then be used as the blueprint for the construction

• f the blocks.

7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.6 Conventional treatment simulation

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.4.6 Slide 7

Acquisition of patient data

After the proper determination of beam geometry, patient

contours may be taken at any plane of interest to be used for treatment planning.

Although more sophisticated devices exist, the simplest

and most widely available method for obtaining a patient contour is through the use of lead wire. 7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.6 Conventional treatment simulation

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.4.6 Slide 8

Acquisition of patient data (cont.) The lead wire method:

The wire is placed on a transverse plane parallel to the

isocenter plane.

Next the wire is shaped to the patient’s contour. The shape of the wire is then transferred to a sheet of

graph paper. 7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.6 Conventional treatment simulation

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.4.6 Slide 9

Acquisition of patient data (cont.)

Use of a special

drawing instrument 7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.6 Conventional treatment simulation

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.4.7 Slide 1

7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.7 Computed tomography-based conventional simulation

Data acquisition with Computed Tomography

With the growing popularity of computed tomography (CT)

in the 1990s, the use of CT scanners in radiotherapy became widespread.

Anatomical information on CT scans is presented in the

form of transverse slices, which contain anatomical images of very high resolution and contrast.

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.4.7 Slide 2

CT images provide excellent soft tissue contrast allowing

for greatly improved tumor localization and definition in comparison to conventional simulation.

Patient contours can be

• btained easily from the

CT data:

• patient’s skin contour
• target
• any organs of interest

7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.7 Computed tomography-based conventional simulation

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.4.7 Slide 3

The position of each slice and therefore the target can be

related to bony anatomical landmarks through the use of scout or pilot images obtained at the time of scanning. 7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.7 Computed tomography-based conventional simulation

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.4.7 Slide 4

Scout films

Pilot or scout films are obtained by keeping the x-ray

source in a fixed position and moving the patient (translational motion) through the stationary slit beam.

The result is a high definition radiograph which is

divergent on the transverse axis, but non-divergent on the longitudinal axis. 7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.7 Computed tomography-based conventional simulation

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.4.7 Slide 5

Scout films

The target position can also be determined through

comparison between the CT scout and pilot films.

Note: A different magnification between simulator film and

scout film must be taken into account.

This procedure allows for a more accurate determination

• f tumor extent and therefore more precise field definition

at the time of simulation. 7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.7 Computed tomography-based conventional simulation

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.4.7 Slide 6

Scout films

If scanned in treatment position, field limits and

shielding parameters can be directly set with respect to the target position, similar to conventional treatment simulation.

The result is that the treatment port more closely conforms

to the target volume, reducing treatment margins around the target and increasing healthy tissue sparing. 7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.7 Computed tomography-based conventional simulation

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.4.8 Slide 1

Virtual Simulation

Virtual simulation is the treatment simulation of patients

based solely on CT information.

The premise of virtual simulation is that the CT data can

be manipulated to render synthetic radiographs of the patient for arbitrary geometries. 7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.8 Computed tomography-based virtual simulation

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.4.8 Slide 2

CT-Simulator

Dedicated CT scanners for use in radiotherapy treatment

simulation and planning have been developed.

They are known as

CT-simulators. Example of a modern CT-simulator 7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.8 Computed tomography-based virtual simulation

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.4.8 Slide 3

The components of a CT-simulator include:

• CT scanner, including scanners with a

large bore (with an opening of up to 85 cm to allow for a larger variety of patient positions and the placement of treatment accessories during CT scanning);

• movable lasers for patient positioning and

marking;

• a flat table top to more closely match

radiotherapy treatment positions;

• a powerful graphics workstation, allowing

for image manipulation and formation.

7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.8 Computed tomography-based virtual simulation

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.4.8 Slide 4

Virtual Simulation

Synthetic radiographs can be produced by tracing ray-

lines from a virtual source position through the CT data of the patient to a virtual film plane and simulating the attenuation of x-rays.

The synthetic radiographs are called

Digitally Reconstructed Radiographs (DRRs).

The advantage of DRRs is that anatomical information

may be used directly in the determination of treatment field parameters. 7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.8 Computed tomography-based virtual simulation

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.4.8 Slide 5

Note: gray levels, brightness, and contrast can be adjusted to provide an optimal image. Example of a DRR 7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.8 Computed tomography-based virtual simulation

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.4.8 Slide 6

Beam’s eye view (BEV) Beam’s eye views (BEV) are projections through the patient onto a virtual film plane perpendicular to the beam direction. The projections include:

the treatment beam axes field limits

• utlined structures

7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.8 Computed tomography-based virtual simulation

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.4.8 Slide 7

Beam’s eye view (BEV)

BEVs are frequently superimposed onto the corresponding

DRRs resulting in a synthetic representation of a simulation radiograph. 7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.8 Computed tomography-based virtual simulation

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.4.8 Slide 8

Multi-planar reconstructions (MPR)

Multi-planar reconstructions (MPR) are images formed

from reformatted CT data.

They are effectively CT images through arbitrary planes

• f the patient.

Although typically sagittal or coronal MPR cuts are used

for planning and simulation, MPR images through any arbitrary plane may be obtained. 7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.8 Computed tomography-based virtual simulation

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.4.9 Slide 1

7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.9 Conventional simulator vs. CT simulator

Advantage Disadvantage

useful to perform a

fluoroscopic simulation in order to verify isocenter position and field limits as well as to mark the patient for treatment

limited soft tissue contrast tumor mostly not visible requires knowledge of

tumor position with respect to visible landmarks

restricted to setting field

limits with respect to bony landmarks or anatomical structures visible with the aid of contrast Conventional simulator

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.4.9 Slide 2

Advantage Disadvantage

increased soft tissue

contrast

axial anatomical

information available

delineation of target and

OARs directly on CT slices

allows DRRs allows BEV limitation in use for some

treatment setups where patient motion effects are involved

require additional training

and qualification in 3D planning CT simulator 7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.9 Conventional simulator vs. CT simulator

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.4.9 Slide 3

Another important advantage of the CT-simulation

process over the conventional simulation process is the fact that the patient is not required to stay after the scanning has taken place.

The patient only stays the minimum time necessary to

acquire the CT data set and mark the position of reference isocenter; this provides the obvious advantage as the radiotherapy staff may take their time in planning the patient as well as try different beam configurations without the patient having to wait on the simulator couch. 7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.9 Conventional simulator vs. CT simulator

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.4.9 Slide 4

Another important advantage : A CT-simulator allows the

user to generate DRRs and BEVs even for beam geometries which were previously impossible to simulate conventionally.

Example:

A DRR with superimposed beam’s eye view for a vertex field of a brain patient. This treatment geometry would be impossible to simulate on a conventional simulator because the film plane is in the patient.

7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.9 Conventional simulator vs. CT simulator

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.4.10 Slide 1

7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.10 Magnetic resonance imaging for treatment planning

MR imaging plays an increasing role in treatment

planning.

The soft tissue contrast offered by magnetic resonance

imaging (MRI) in some areas, such as the brain, is superior to that of CT, allowing small lesions to be seen with greater ease.

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.4.10 Slide 2

Disadvantage of MRI It cannot be used for radiotherapy simulation and planning for several reasons:

• The physical dimensions of the MRI and its accessories limit the use
• f immobilization devices and compromise treatment positions.
• Bone signal is absent and therefore digitally reconstructed

radiographs cannot be generated for comparison to portal films.

• There is no electron density information available for heterogeneity

corrections on the dose calculations.

• MRI is prone to geometrical artifacts and distortions that may affect

the accuracy of the treatment.

7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.10 Magnetic resonance imaging for treatment planning

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.4.10 Slide 3

To overcome this problem, many modern virtual

simulation and treatment planning systems have the ability to combine the information from different imaging studies using the process of image fusion or registration.

CT-MR image registration or fusion combines the

• accurate volume definition from MR

with

• electron density information available from CT.

7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.10 Magnetic resonance imaging for treatment planning

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.4.10 Slide 4

On the left is an MR image of a patient with a brain tumour. The target has been outlined and the result was superimposed on the patient’s CT scan. Note that the particular target is clearly seen on the MR image but only portions of it are observed on the CT scan.

MR CT

7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.10 Magnetic resonance imaging for treatment planning

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.4.11 Slide 1

7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.11 Summary of simulation procedures

Goals and tools in conventional and CT simulation

from CT data manual Contour acquisition: conformal to target bony landmarks Shielding design: BEV/DRR fluoroscopy Determination of beam geometry: from CT data bony landmarks Identification of target volume: pilot/scout views fluoroscopy Treatment position: CT simulation Conventional Goals

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.4.11 Slide 2

The following six steps are typically involved in conventional simulation procedures:

(1) Determination of patient treatment position with

fluoroscopy

(2) Determination of beam geometry (3) Determination field limits and isocenter (4) Acquisition of contour (5) Acquisition of beam’s eye view and set-up radiographs (6) Marking of patient

7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.11 Summary of simulation procedures

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.4.11 Slide 3

The following nine steps are typically involved in CT simulation procedures:

(1) Determination of patient treatment position with pilot/scout films (2) Determination and marking of reference isocenter (3) Acquisition of CT data and transfer to virtual simulation workstation (4) Localization and contouring of targets and critical structures (5) Determination treatment isocenter with respect to target and

reference isocenter.

(6) Determination of beam geometry (7) Determination of field limits and shielding (8) Transfer of CT and beam data to treatment planning system (9) Acquisition of beam’s eye view and setup DRRs

7.4 PATIENT DATA ACQUISITION AND SIMULATION

7.4.11 Summary of simulation procedures

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.5 Slide 1

7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS Clinical considerations for photon beams include the following items:

Isodose curves Wedge filters Bolus Compensating filters Corrections for contour irregularities Corrections for tissue inhomogeneities Beam combinations and clinical application

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.5.1 Slide 1

7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.1 Isodose curves

Isodose curves are defined as lines that join points of

equal dose.

They offer a planar representation of the dose

distribution.

Isodose curves are useful to characterize the behavior of

• one beam
• a combination of beams
• beams with different shielding
• wedges
• bolus, etc.

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.5.1 Slide 2

How isodose curves can be obtained?

They can be measured directly using a beam scanning

device in a water phantom.

They can be calculated from percentage depth dose and

beam profile data.

They can be adopted from an atlas for isodose curves.

7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.1 Isodose curves

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.5.1 Slide 3

To which dose values isodose curves can refer?

While isodose curves can be made to display the actual

dose in Gy (per fraction or total dose), it is more common to present them normalized to 100% at a fixed point.

Possible point normalizations are:

• Normalization to 100% at the depth of dose

maximum on the central axis;

• Normalization at the isocenter;
• Normalization at the point of dose prescription.

7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.1 Isodose curves

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.5.1 Slide 4

Different normalizations for a single 18 MV photon beam incident on a patient contour

Isodose curves for a fixed SSD beam normalized at depth of dose maximum Isodose curves for an isocentric beam normalized at the isocenter

7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.1 Isodose curves

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.5.2 Slide 1

7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.2 Wedge filters

Three types of wedge filters are currently in use:

(1) Physical (requiring manual intervention) (2) Motorized (3) Dynamic Physical wedge:

It is an angled piece of lead or steel that is placed in the beam to produce a gradient in radiation intensity.

Motorized wedge:

It is a similar physical device, integrated into the head of the unit and controlled remotely.

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.5.2 Slide 2

• Physical wedge:

A set of wedges (15°, 30°, 45°, and 60°) is usually provided with the treatment machine. 7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.2 Wedge filters

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.5.2 Slide 3

A dynamic wedge produces the same wedged intensity

gradient by having one jaw close gradually while the beam is on. 7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.2 Wedge filters

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.5.2 Slide 4

Isodose curves obtained for a wedged 6 MV photon beam. The isodoses have been normalized to zmax with the wedge in place. 7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.2 Wedge filters

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.5.2 Slide 5

The wedge angle is

defined as the angle between the 50% isodose line and the perpendicular to the beam central axis.

Wedge angles in the

range from 10° to 60° are commonly available. 7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.2 Wedge filters

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.5.2 Slide 6

(1) Wedges can be used to

compensate for a sloping surface.

Example 1: Two 15° wedges are used in a nasopharyngeal treatments to compensate for the decreased thickness anteriorly.

There are two main uses of wedges 7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.2 Wedge filters

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.5.2 Slide 7

(1) Wedges can be used to

compensate for a sloping surface.

Example 2: A wedged pair of beams is used to compensate for the hot spot that would be produced with a pair of open beams at 90° to each other.

7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.2 Wedge filters

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.5.2 Slide 8

(2) Wedges can also be used in the treatment of relatively low

lying lesions where two beams are placed at an angle (less than 180°) called the hinge angle. The optimal wedge angle (assuming a flat patient surface) may be estimated from: There are two main uses of wedges ( cont.)

= ° − wedge angle 90 hinge angle

7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.2 Wedge filters

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.5.2 Slide 9

Example:

• A wedge pair of 6 MV beams

incident on a patient.

• The hinge angle is 90°

(orthogonal beams) for which the optimal wedge angle would be 45°.

• However, in this case the

additional obliquity of the surface requires the use of a higher wedge angle of 60°.

7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.2 Wedge filters

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.5.2 Slide 10

Wedge factor

The wedge factor is defined as the ratio of dose at a

specified depth (usually zmax) on the central axis with the wedge in the beam to the dose under the same conditions without the wedge.

This factor is used in monitor unit calculations to

compensate for the reduction in beam transmission produced by the wedge.

The wedge factor depends on depth and field size.

7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.2 Wedge filters

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.5.3 Slide 1

(1) Increase of the surface

dose

Because of the dose buildup in megavoltage beams between the surface and the dose maximum (at a certain depth zmax), the dose may not be sufficient for superficial targets.

Bolus is a tissue-equivalent material placed in contact with the skin to achieve one or both of the following:

dose depth

7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.3 Bolus

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.5.3 Slide 2

To increase the surface dose, a layer of uniform

thickness bolus is often used (0.5 –1.5 cm), since it does not significantly change the shape of the isodose curves at depth.

Several flab-like materials were developed commercially

for this purpose.

Cellophane wrapped wet towels or gauze offer a low cost

substitute. 7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.3 Bolus

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.5.3 Slide 3

(2) Compensation for

missing tissue

A custom made bolus can be built such that it conforms to the patient skin on one side and yields a flat perpendicular incidence to the beam.

Bolus is also used to achieve:

wax bolus

7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.3 Bolus

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.5.3 Slide 4

The result is an isodose distribution that is identical to that

produced on a flat phantom.

However, skin sparing is not maintained with a bolus,

in contrast to the use of a compensator. 7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.3 Bolus

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.5.3 Slide 5

Difference between a bolus and a compensating filter:

a)

A wax bolus is used. Skin sparing is lost with bolus.

b)

A compensator achieving the same dose distribution as in (a) is constructed and attached to the treatment unit. Due to the large air gap skin sparing is maintained.

7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.3 Bolus

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.5.4 Slide 1

7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.4 Compensating filters

A compensating filter achieves the same effect on the

dose distribution as a shaped bolus but does not cause a loss of skin sparing.

Compensating filters can be made of almost any material,

but metals such as lead are the most practical and compact.

Compensating filters can produce a gradient in two

dimensions.

They are usually placed in a shielding slot on the

treatment unit head.

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.5.4 Slide 2

Thickness of the compensator is determined on a point-

by-point basis depending on the fraction I/Io of the dose without a compensator which is required at a certain depth in the patient.

The thickness of compensator x along the ray line above

that point can be solved from the attenuation law: where μ is the linear attenuation coefficient for the radiation beam and material used to construct the compensator.

I I e

x −μ

=

7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.4 Compensating filters

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.4.9 Slide 3

Advantage Disadvantage

preservation of the skin

sparing effect

generally more laborious

and time consuming

difficult to calculate resulting

dose distribution

additional measurements

may be required Use of Compensating Filters 7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.4 Compensating filters

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.5.5 Slide 1

7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.5 Corrections for contour irregularities

Measured dose distributions apply to a flat radiation

beam incident on a flat homogeneous water phantom.

To relate such measurements to the actual dose

distribution in a patient, corrections for irregular surface and tissue inhomogeneities have to be applied.

Three methods for contour correction are used: (1) the (manual) isodose shift method; (2) the effective attenuation coefficient method; (3) the TAR method.

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.5.5 Slide 2

• Grid lines are drawn parallel

to the central beam axis all across the field.

• The tissue deficit (or excess) h is

the difference between the SSD along a gridline and the SSD on the central axis.

• k is an energy dependent

parameter given in the next slide.

• The isodose distribution for a flat

phantom is aligned with the SSD central axis on the patient contour.

• For each gridline, the overlaid

isodose distribution is shifted up (or down) such that the overlaid SSD is at a point k×h above (or below) the central axis SSD.

(1) Manual isodose shift method 7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.5 Corrections for contour irregularities

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.5.5 Slide 3

0.4 > 30 0.5 15 – 30 0.6 5 – 15 0.7

60Co - 5

0.8 < 1 k (approximate) Photon energy (MV)

Parameter k used in the isodose shift method 7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.5 Corrections for contour irregularities

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.5.5 Slide 4

(2) Effective attenuation coefficient method

The correction factor is determined from the attenuation

factor exp(-μx), where x is the depth of missing tissue above the calculation point, and μ is the linear attenuation coefficient of tissue for a given energy.

For simplicity the factors are usually pre-calculated and

supplied in graphical or tabular form. 7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.5 Corrections for contour irregularities

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.5.5 Slide 5

(3) TAR method

The tissue-air ratio (TAR) correction method is also based

• n the attenuation law, but takes the depth of the

calculation point and the field size into account.

Generally, the correction factor CF as a function of depth

z, thickness of missing tissue h, and field size f, is given by:

F

( , ) ( , ) TAR z h f C TAR z f − =

7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.5 Corrections for contour irregularities

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.5.6 Slide 1

7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.6 Corrections for tissue inhomogeneities

In a simple approach to calculate the dose and its

distribution in a patient, one may assume that all tissues are water-equivalent.

However, in the actual patient the photon beam traverses

tissues with varying densities and atomic numbers such as fat, muscle, lung, air, and bone.

This will influence the attenuation and scatter of photons

beam such that the depth dose curve will deviate from that in water.

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.5.6 Slide 2

Tissues with densities and atomic numbers different from

those of water are referred to as tissue inhomogeneities

• r heterogeneities.

Inhomogeneities in the patient result in:

• Changes in the absorption of the primary beam and

associated scattered photons

• Changes in electron fluence.

The importance of each effect depends on the position of

the point of interest relative to the inhomogeneity. 7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.6 Corrections for tissue inhomogeneities

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.5.6 Slide 3

Difference in the isodose curves obtained using a single vertical 7x7cm2 field.

• Top:

Assuming that all tissues (including the lung) have water-equivalent density

• Bottom:

Taking into account the real tissue density 7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.6 Corrections for tissue inhomogeneities

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.5.6 Slide 4

In the megavoltage range the Compton interaction

dominates and its cross-section depends on the electron density (in electrons per cm3).

The following four methods correct for the presence of

inhomogeneities within certain limitations:

• TAR method
• Batho power law method
• equivalent TAR method
• isodose shift method

7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.6 Corrections for tissue inhomogeneities

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.5.6 Slide 5

The four methods are presented using the schematic

diagram which shows an inhomogeneity with an electron density ρe nested between two layers of water-equivalent tissue.

z1 z2 z3 Point P ρ1 = 1 ρ2 = ρe ρ3 = 1

7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.6 Corrections for tissue inhomogeneities

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.5.6 Slide 6

TAR method The dose at each point is corrected by the factor CF: where z’ = z1 + ρez2 + z3 and z = z1 + z2 +z3

d F d

( ', ) ( , ) TAR z r C TAR z r =

z1 z2 z3 Point P ρ1 = 1 ρ2 = ρe ρ3 = 1

7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.6 Corrections for tissue inhomogeneities

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Batho Power-law method The dose at each point is corrected by: where z’ = z1 + ρ2z2 + z3 and z = z1 + z2 +z3

3 2 2

d F 1 d

( ', ) ( , ) TAR z r C TAR z r

ρ −ρ −ρ

=

z1 z2 z3 Point P ρ1 = 1 ρ2 = ρe ρ3 = 1

7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.6 Corrections for tissue inhomogeneities

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.5.6 Slide 8

Equivalent TAR method It is similar to the TAR method. The field size parameter rd is now modified into r'd as a function of density where z’ = z1 + ρ2z2 + z3 and z = z1 + z2 +z3

d F d

( ', ' ) ( , ) TAR z r C TAR z r =

z1 z2 z3 Point P ρ1 = 1 ρ2 = ρe ρ3 = 1

7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.6 Corrections for tissue inhomogeneities

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Isodose shift method

• The isodose shift method for the dose correction due to the

presence of inhomogeneities is essentially identical to the isodose shift method outlined in the previous section for contour irregularities.

• Isodose shift factors for several types of tissue have been

determined for isodose points beyond the inhomogeneity.

• The factors are energy dependent but do not vary significantly with

field size.

• The factors for the most common tissue types in a 4 MV photon

beam are: air cavity: -0.6; lung: -0.4; and hard bone: 0.5. The total isodose shift is the thickness of inhomogeneity multiplied by the factor for a given tissue. Isodose curves are shifted away from the surface when the factor is negative.

7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.6 Corrections for tissue inhomogeneities

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.5.7 Slide 1

7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.7 Beam combinations and clinical application

Single photon beams

are of limited use in the treatment of deep- seated tumors, since they give a higher dose near the entrance at the depth of dose maximum than at depth.

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Single fields are often used for palliative treatments or

for relatively superficial lesions (depth < 5-10 cm, depending on the beam energy).

For deeper lesions, a combination of two or more photon

beams is usually required to concentrate the dose in the target volume and spare the tissues surrounding the target as much as possible. 7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.7 Beam combinations and clinical application

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.5.7 Slide 3

Weighting and normalization

Dose distributions for multiple beams can be normalized

to 100% just as for single beams:

• at zmax for each beam,
• at isocenter for each beam.

This implies that each beam is equally weighted.

7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.7 Beam combinations and clinical application

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Weighting and normalization

A beam weighting may additionally applied at the

normalization point for the given beam.

Example:

A wedged pair with zmax normalization weighted as 100 : 50% will show one beam with the 100% isodose at zmax and the

• ther one with 50% at zmax.

A similar isocentric weighted beam pair would show the

150% isodose at the isocenter. 7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.7 Beam combinations and clinical application

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.5.7 Slide 5

Fixed SSD vs. isocentric techniques

Fixed SSD techniques require

adjusting the patient such that the skin is at the correct distance (nominal SSD) for each beam orientation.

Isocentric techniques require placing

the patient such that the target (usually) is at the isocenter.

The machine gantry is then rotated

around the patient for each treatment field. 7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.7 Beam combinations and clinical application

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• There is little difference between fixed SSD techniques

and isocentric techniques with respect to the dose:

• Fixed SSD arrangements are usually at a greater

SSD than isocentric beams because the machine isocenter is on the patient skin.

• They have therefore a slightly higher PDD at depth.
• Additionally, beam divergence is smaller with SSD

due to the larger distance. Fixed SSD vs. isocentric techniques 7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.7 Beam combinations and clinical application

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.5.7 Slide 7

• These dosimetric advantages of SSD techniques are

small.

• With the exception of very large fields exceeding

40x40 cm2, the advantages of using a single set-up point (i.e., the isocenter) greatly outweigh the dosimetric advantage of SSD beams. Fixed SSD vs. isocentric techniques 7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.7 Beam combinations and clinical application

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7.5 Clinical considerations for photon beams

7.5.7 Beam combinations and clinical application

Parallel opposed beams

Example:

A parallel-opposed beam pair is incident on a patient.

Note the large rectangular

area of relatively uniform dose (<15% variation).

The isodoses have been

normalized to 100% at the isocenter.

This beam combination is well suited to a large variety of

treatment sites (e.g., lung, brain, head and neck).

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.5.7 Slide 9

Multiple co-planar beams

Multiple coplanar beams allows for a higher dose in the

beam intersection region. Two examples: 4-field box 3-field technique using wedges 7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.7 Beam combinations and clinical application

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Multiple co-planar beams 4-field box

A 4-field box allows

for a very high dose to be delivered at the intersection of the beams. 7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.7 Beam combinations and clinical application

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.5.7 Slide 11

Multiple co-planar beams 3-field technique using wedges

A 3-field technique

requires the use of wedges to achieve a similar result.

Note that the latter can

produce significant hot spots near the entrance

• f the wedged beams

and well outside the targeted area. 7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.7 Beam combinations and clinical application

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.5.7 Slide 12

Multiple co-planar beams: General characteristics

similar indications The high dose area has a rhombic shape Opposing pairs at angles other than 90° treatments in the pelvis, where most lesions are central (e.g., prostate, bladder, uterus). Produces a relatively high dose box shaped region 4-field box low-lying lesions (e.g., maxillary sinus and thyroid lesions). Used to achieve a trapezoid shaped high dose region Wedge pairs Used for: Characteristics Type

7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.7 Beam combinations and clinical application

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.5.7 Slide 13

Multiple co-planar beams: General characteristics

• Wedge pair:

Two beams with wedges (often orthogonal) are used to achieve a trapezoid shaped high dose region. This technique is useful in relatively low-lying lesions (e.g., maxillary sinus and thyroid lesions).

• 4-field box:

A technique of four beams (two opposing pairs at right angles) producing a relatively high dose box shaped region. The region of highest dose now occurs in the volume portion that is irradiated by all four fields. This arrangement is used most often for treatments in the pelvis, where most lesions are central (e.g., prostate, bladder, uterus).

• Opposing pairs at angles other than 90°:

also result in the highest dose around the intersection of the four beams, however, the high dose area here has a rhombic shape.

7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.7 Beam combinations and clinical application

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.5.7 Slide 14

Multiple co-planar beams: General characteristics

Occasionally, three sets of opposing pairs are used,

resulting in a more complicated dose distribution, but also in a spread of the dose outside the target over a larger volume, i.e., in more sparing of tissues surrounding the target volume.

The 3-field box technique is similar to a 4-field box

• technique. It is used for lesions that are closer to the

surface (e.g., rectum). Wedges are used in the two

• pposed beams to compensate for the dose gradient in

the third beam. 7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.7 Beam combinations and clinical application

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.5.7 Slide 15

Rotational techniques

Isodose curves for

two bilateral arcs

• f 120° each.

Note:

The isodoses are tighter along the angles avoided by the arcs (anterior and posterior). 7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.7 Beam combinations and clinical application

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.5.7 Slide 16

Rotational techniques: General characteristics

The target is placed at the isocenter, and the machine

gantry is rotated about the patient in one or more arcs while the beam is on.

Rotational techniques produce a relatively concentrated

region of high dose near the isocenter.

But they also irradiate a greater amount of normal tissue

to lower doses than fixed-field techniques. 7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.7 Beam combinations and clinical application

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.5.7 Slide 17

Rotational techniques: General characteristics

Useful technique used mainly for prostate, bladder, cervix

and pituitary lesions, particularly boost volumes.

The dose gradient at the edge of the field is not as sharp

as for multiple fixed field treatments.

Skipping an angular region during the rotation allows the

dose distribution to be pushed away from the region; however, this often requires that the isocentre be moved closer to this skipped area so that the resulting high-dose region is centered on the target . 7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.7 Beam combinations and clinical application

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.5.7 Slide 18

Multiple non-coplanar beams: General characteristics

Non-coplanar beams

arise from non-standard couch angles coupled with gantry angulations. 7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.7 Beam combinations and clinical application

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.5.7 Slide 19

Multiple non-coplanar beams: General characteristics

Non-coplanar beams may be useful to get more adequate

critical structure sparing compared to conventional co- planar beam arrangement.

Dose distributions from non-coplanar beam combinations

yield similar dose distributions to conventional multiple field arrangements. 7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.7 Beam combinations and clinical application

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.5.7 Slide 20

Multiple non-coplanar beams: General characteristics

Care must be taken when planning the use of non-

coplanar beams to ensure no collisions occur between the gantry and patient or couch.

Non-coplanar beams are most often used for treatments

• f brain as well as head and neck disease where the

target volume is frequently surrounded by critical structures. 7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.7 Beam combinations and clinical application

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.5.7 Slide 21

Multiple non-coplanar beams: General characteristics

Non-coplanar arcs are also used. The best-known example is

the multiple non-coplanar converging arcs technique used in radiosurgery. 7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.7 Beam combinations and clinical application

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.5.7 Slide 22

Field matching

Field matching at the skin is

the easiest field matching technique.

However, due to beam

divergence, this will lead to significant overdosing of tissues at depth and is only used in regions where tissue tolerance is not compromised. 7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.7 Beam combinations and clinical application

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.5.7 Slide 23

Field matching

For most clinical situations field matching is performed

at depth rather than at the skin.

To produce a junction dose

similar to that in the center

• f the open fields, beams must

be matched such that their diverging edges match at the desired depth z.

50% isodose lines

z 7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.7 Beam combinations and clinical application

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.5.7 Slide 24

Field matching

For two adjacent fixed SSD fields of different lengths L1

and L2, the surface gap g required to match the two fields at a depth z is:

1 2

0.5 0.5 z g L SSD z L SSD ⎛ ⎞ = ⋅ ⋅⎜ ⎟ ⎝ ⎠ ⎛ ⎞ + ⋅ ⋅⎜ ⎟ ⎝ ⎠

7.5 CLINICAL CONSIDERATIONS FOR PHOTON BEAMS

7.5.7 Beam combinations and clinical application

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.6 Slide 1

7.6 TREATMENT PLAN EVALUATION

It is essential to assess the "quality" of a treatment plan

regardless whether the dose calculations are performed

• n computer
• r by hand.

Good "quality" means that the calculated dose distribution

• f the treatment plan complies with he clinical aim of the

treatment.

A radiation oncologist must therefore evaluate the result

• f the treatment plan.

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.6 Slide 2

Depending on the method of calculation, the dose

distribution may be obtained:

(1) Only for a few significant points within the target

volume;

(2) For a two-dimensional grid of points over a contour or

an image;

(3) For a full three-dimensional array of points that cover

the patient’s anatomy. 7.6 TREATMENT PLAN EVALUATION

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.6 Slide 3

The treatment plan evaluation generally consists of

verifying:

• the treatment portals

They are verified to ensure that the desired PTV is covered adequately.

• the isodose distribution

It is verified to ensure that target coverage is adequate and that critical structures surrounding the PTV are spared as necessary. 7.6 TREATMENT PLAN EVALUATION

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.6 Slide 4

The following tools are used in the evaluation of the planned dose distribution:

Isodose curves Orthogonal planes and isodose surfaces Dose distribution statistics Differential Dose Volume Histogram Cumulative Dose Volume Histogram

These tools are explained in the following slides. 7.6 TREATMENT PLAN EVALUATION

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.6.1 Slide 1

7.6 TREATMENT PLAN EVALUATION

7.6.1 Isodose curves

Isodose curves are used to evaluate treatment plans

along a single plane or over several planes in the patient. Example: The isodose covering the periphery of the target is compared to the isodose at the isocenter.

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.6.1 Slide 2

Same example: The isodose line through the ICRU reference point is 152%. The maximum dose 154%. The 150% isodose curve completely covers the PTV. [%]

• 150
• 140
• 130
• 120
• 100
• 70
• 50

7.6 TREATMENT PLAN EVALUATION

7.6.1 Isodose curves

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.6.1 Slide 3

If the ratio of isodoses covering the periphery of the

target to that at the isocenter is within a desired range (e.g., 95-100%) then the plan may be acceptable provided critical organ doses are not exceeded.

This approach is ideal if the number of transverse

slices is small. 7.6 TREATMENT PLAN EVALUATION

7.6.1 Isodose curves

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.6.2 Slide 1

7.6 TREATMENT PLAN EVALUATION

7.6.2 Orthogonal planes and isodose surfaces

When a larger number of transverse planes are used for

calculation it may be impractical to evaluate the plan on the basis of axial slice isodose distributions alone.

In such cases, isodose distributions can also be

generated on orthogonal CT planes, reconstructed from the original axial data.

For example, sagittal and coronal plane isodose

distributions are usually available on most 3D treatment planning systems.

Displays on arbitrary oblique planes are also becoming

increasingly common.

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.6.2 Slide 2

An alternative way to display isodoses is to map them in

three dimensions and overlay the resulting isosurface on a 3D display featuring surface renderings of the target and

• r/other organs.

7.6 TREATMENT PLAN EVALUATION

7.6.2 Orthogonal planes and isodose surfaces

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.6.2 Slide 3

Example: Prostate cancer Target volume: blue Prescription isodose: white wireframe Bladder and rectum are also shown. 7.6 TREATMENT PLAN EVALUATION

7.6.2 Orthogonal planes and isodose surfaces

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.6.2 Slide 4

Such displays are useful to assess target coverage in a qualitative manner. Disadvantage:

They do not convey a sense of distance between the

isosurface and the anatomical volumes.

They do not give a quantitative volume information.

7.6 TREATMENT PLAN EVALUATION

7.6.2 Orthogonal planes and isodose surfaces

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.6.3 Slide 1

7.6 TREATMENT PLAN EVALUATION

7.6.3 Dose statistics

In order to get more quantitative information, statistics

tools have been introduced.

In contrast to the isodose tools, the dose statistics tools

cannot show the spatial distribution of dose superimposed

• n CT slices or anatomy that has been outlined based on

CT slices.

Instead, they can provide quantitative information on the

volume of the target or critical structure, and on the dose received by that volume.

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.6.3 Slide 2

From the location of matrix points within an organ and the calculated doses at these points, a series of statistical characteristics can be obtained. These include:

Minimum dose to the volume Maximum dose to the volume Mean dose to the volume Dose received by at least 95% of the volume Volume irradiated to at least 95% of the prescribed dose.

7.6 TREATMENT PLAN EVALUATION

7.6.3 Dose statistics

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.6.3 Slide 3

Target dose statistics as well as organ dose statistics can

be performed.

The "Dose received by at least 95% of the volume" and

the "Volume irradiated to at least 95% of the prescribed dose" are only relevant for the target volume.

Organ dose statistics are especially useful in dose

reporting, since they are simpler to include in a patient chart than dose-volume histograms that are described in the next slides. 7.6 TREATMENT PLAN EVALUATION

7.6.3 Dose statistics

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.6.4 Slide 1

7.6 TREATMENT PLAN EVALUATION

7.6.4 Dose-volume histograms

Dose volume histograms (DVHs) summarize the

information contained in a three-dimensional treatment plan.

This information consists of dose distribution data over a

three-dimensional matrix of points over the patient’s anatomy.

DVHs are extremely powerful tools for quantitative

evaluation of treatment plans.

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.6.4 Slide 2

In its simplest form a DVH represents a frequency

distribution of dose values within a defined volumes such as:

• the PTV itself
• a specific organ

in the vicinity of the PTV. frequency dose value 7.6 TREATMENT PLAN EVALUATION

7.6.4 Dose-volume histograms

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.6.4 Slide 3

Rather than displaying the frequency, DVHs are usually

displayed in the form of “per cent volume of total volume” on the

• rdinate against

the dose on the abscissa.

per cent volume of total volume

dose value in Gy 7.6 TREATMENT PLAN EVALUATION

7.6.4 Dose-volume histograms

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.6.4 Slide 4

Two types of DVHs are in use:

Direct (or differential) DVH Cumulative (or integral) DVH

Definition: The volume that receives at least the given dose and plotted versus dose. 7.6 TREATMENT PLAN EVALUATION

7.6.4 Dose-volume histograms

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.6.4 Slide 5

Direct Dose Volume Histogram

To create a direct DVH, the computer sums the number of

voxels which have a specified dose range and plots the resulting volume (or the percentage of the total organ volume) as a function of dose.

The ideal DVH for a target volume would be a single

column indicating that 100% of the volume receives the prescribed dose.

For a critical structure, the DVH may contain several

peaks indicating that different parts of the organ receive different doses. 7.6 TREATMENT PLAN EVALUATION

7.6.4 Dose-volume histograms

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.6.4 Slide 6

Example: Prostate cancer target rectum Differential DVHs 7.6 TREATMENT PLAN EVALUATION

7.6.4 Dose-volume histograms

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.6.4 Slide 7

Cumulative Dose Volume Histogram

Traditionally, physicians have sought to answer questions

such as: “How much of the target is covered by the 95% isodose line?”

In 3-D treatment planning this question is equally relevant

and the answer cannot be extracted directly from the direct DVH, since it would be necessary to determine the area under the curve for all dose levels above 95% of the prescription dose. 7.6 TREATMENT PLAN EVALUATION

7.6.4 Dose-volume histograms

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.6.4 Slide 8

Example: Prostate cancer Target Critical structure: rectum Integral DVHs 7.6 TREATMENT PLAN EVALUATION

7.6.4 Dose-volume histograms

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.6.4 Slide 9

For this reason, cumulative DVH displays are more popular.

The computer calculates the volume of the target (or

critical structure) that receives at least the given dose and plots this volume (or percentage volume) versus dose.

All cumulative DVH plots start at 100% of the volume for

zero dose, since all of the volume receives at least no dose. 7.6 TREATMENT PLAN EVALUATION

7.6.4 Dose-volume histograms

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.6.4 Slide 10

While displaying the percent volume versus dose is more

popular, it is also useful in some circumstances to plot the absolute volume versus dose.

For example, if a CT scan does not cover the entire

volume of an organ such as the lung and the un-scanned volume receives very little dose, then a DVH showing percentage volume versus dose for that organ will be biased, indicating that a larger percentage of the volume receives dose.

Furthermore, in the case of some critical structures,

tolerances are known for irradiation of fixed volumes specified in cm3. 7.6 TREATMENT PLAN EVALUATION

7.6.4 Dose-volume histograms

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.6.4 Slide 11

The main drawback of the DVHs is the loss of spatial

information that results from the condensation of data when DVHs are calculated. 7.6 TREATMENT PLAN EVALUATION

7.6.4 Dose-volume histograms

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.6.5 Slide 1

7.6 TREATMENT PLAN EVALUATION

7.6.5 Treatment evaluation

Port films

A port film is usually an

emulsion-type film, often still in its light-tight paper envelope, that is placed in the radiation beam beyond the patient. Since there is no conversion of x rays to light photons as in diagnostic films, the films need not be removed from its envelope.

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.6.5 Slide 2

Port films

Two port films are available. Depending on their sensitivity (or speed) port films can be

used for:

• Localization:

A fast film is placed in each beam at the beginning or end of the treatment to verify that the patient installation is correct for the given beam.

• Verification:

A slow film is placed in each beam and left there for the duration of the treatment. 7.6 TREATMENT PLAN EVALUATION

7.6.5 Treatment evaluation

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.6.5 Slide 3

Advantage Disadvantage

Fast films generally

produce a better image

Recommended for

verifying small or complex beam arrangements

Patient or organ

movement during treatment will not affect the quality of the film

Not recommended for larger

fields for example where as many as 4 films may be required to verify the treatment delivery Localization (fast) vs. verification (slow) films 7.6 TREATMENT PLAN EVALUATION

7.6.5 Treatment evaluation

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.6.5 Slide 4

Localization films used in radiotherapy do not require

intensifying screens such as those used in diagnostic radiology.

Instead, a single thin layer of a suitable metal (such as

copper or aluminum) is used in front of the film (beam entry side) to provide for electronic buildup that will increase the efficiency of the film.

A backing layer is sometimes used with double emulsion

films to provide backscatter electrons. 7.6 TREATMENT PLAN EVALUATION

7.6.5 Treatment evaluation

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.6.5 Slide 5

Port films can be taken either in single or double exposure techniques.

Single exposure:

The film is irradiated with the treatment field alone. This technique is well suited to areas where the anatomical features can clearly be seen inside the treated field. Practically all verification films are single exposure.

Double exposure:

• The film is irradiated with the treatment field first.
• Then the collimators are opened to a wider setting, all shielding

is removed, and a second exposure is given to the film.

• The resulting image shows the treated field and the surrounding

anatomy that may be useful in verifying the beam position.

7.6 TREATMENT PLAN EVALUATION

7.6.5 Treatment evaluation

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.6.5 Slide 6

Double exposure technique: Two examples 7.6 TREATMENT PLAN EVALUATION

7.6.5 Treatment evaluation

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.6.5 Slide 7

Online portal imaging

• Online portal imaging systems consist of
• a suitable radiation detector, usually

attached through a manual or semi-robotic arm to the linac,

• a data acquisition system capable of

transferring the detector information to a computer,

• Software that will process it and convert

it to an image.

• These systems use a variety of detectors,
• all producing computer based images of

varying degrees of quality.

7.6 TREATMENT PLAN EVALUATION

7.6.5 Treatment evaluation

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.6.5 Slide 8

Online portal imaging systems currently include:

(1) Fluoroscopic detectors (2) Ionisation chamber detectors (3) Amorphous silicon detectors

7.6 TREATMENT PLAN EVALUATION

7.6.5 Treatment evaluation

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Fluoroscopic portal imaging detectors:

• work on the same principle as a simulator image intensifier

system.

• The detector consists of a combination of a metal plate and

fluorescent phosphor screen, a 45° mirror and a television camera.

• The metal plate converts incident x-rays to electrons and the

fluorescent screen converts electrons to light photons.

• The mirror deflects light to the TV camera, reducing the length of

the imager, and the TV camera captures a small fraction (<0.1%)

• f the deflected light photons to produce an image.
• Good spatial resolution (depends on phosphor thickness).
• Only a few MU are required to produce an image.
• Uses technology that has been used in many other fields.

7.6 TREATMENT PLAN EVALUATION

7.6.5 Treatment evaluation

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.6.5 Slide 10

Matrix ionisation chamber detectors:

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7.6.5 Treatment evaluation

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Matrix ionisation chamber detectors:

• are based on grid of ion chamber-type electrodes that measure

ionisation from point to point

• The detector consists of two metal plates, 1 mm apart with the

gap filled with isobutene. Each plate is divided into 256 electrodes and the plates are oriented such that the electrodes in

• ne plate are at 90° to the electrodes in the other.
• A voltage is applied between two electrodes across the gap and

the ionisation at the intersection is measured. By selecting each electrode on each plate in turn, a 2D ionisation map is obtained and converted to a grayscale image of 256 x 256 pixels.

• The maximum image size is usually smaller than for fluoroscopic

systems.

7.6 TREATMENT PLAN EVALUATION

7.6.5 Treatment evaluation

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.6.5 Slide 12

Amorphous silicon detectors:

• Solid-state detector array consisting of amorphous silicon

photodiodes and field-effect transistors arranged in a large rectangular matrix.

• Uses metal plate/fluorescent phosphor screen combination like

the fluoroscopic systems. Light photons produce electron-hole pairs in the photodiodes whose quantity is proportional to the intensity allowing an image to be obtained.

• Produces an image with a greater resolution and contrast than

the other systems.

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7.6.5 Treatment evaluation

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.7 Slide 1

7.7 TREATMENT TIME AND MONITOR UNIT CALCULATIONS Introductional remark The process of treatment planning and optimization may be considered as completed if the calculated relative dose distribution shows an acceptable agreement with the PTV. As an example, the 80% isodose curve may well encompasses the PTV. It remains to determine the most important final parameter which controls the absolute dose delivery, that is:

the treatment time (for radiation sources)

• r the

the monitor units (for linacs)

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.7 Slide 2

Data on treatment time and/or monitor units are usually provided by modern TPS after having passed the "dose prescription" procedure. However, a manual calculation method to obtain such data independent from the TPS is of highest importance.

Accidents radiotherapy are

really happening! 7.7 TREATMENT TIME AND MONITOR UNIT CALCULATIONS

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Before going into the details of manual calculation methods for an individual plan, a clear understanding of the following associated issues is required:

• The techniques used for patient setup:
• fixed SSD setup
• isocentric setup
• The methods used for :
• dose prescription
• adding the dose from multiple fields.
• the formulas used for central axis dose calculations

7.7 TREATMENT TIME AND MONITOR UNIT CALCULATIONS

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.7 Slide 4

Methods used for patient setup: (already shown previously)

The patient treatments are carried out either with a fixed

SSD or isocentric technique.

Each of the two techniques is characterized with a specific

dose distribution and treatment time or monitor unit calculation. 7.7 TREATMENT TIME AND MONITOR UNIT CALCULATIONS

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fixed SSD technique isocentric technique The fixed SSD technique results in an isodose distribution that is typically governed by percentage depth dose data. The isocentric technique results in an dose dis- tribution that is typically governed by tissue- maximum ratios (or tissue-phantom ratios). 7.7 TREATMENT TIME AND MONITOR UNIT CALCULATIONS

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.7 Slide 6

Methods used for dose prescription The determination of treatment time or monitor units (whether by the treatment planning system or manually) is directly related to the two following actions:

Selection of an appropriate point for dose prescription

(recommended by ICRU: the ICRU reference point)

Prescription of an absolute dose at this point

7.7 TREATMENT TIME AND MONITOR UNIT CALCULATIONS

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.7 Slide 7

Isodose distributions of a three field treatment of the prostate using fixed SSD on a 6 MV linac

ICRU point

[%]

• 150
• 140
• 130
• 120
• 100
• 70
• 50

The ICRU point

is located at the intersection of three fields.

A dose of 200

cGy per fraction is prescribed at the ICRU point.

Example:

7.7 TREATMENT TIME AND MONITOR UNIT CALCULATIONS

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.7 Slide 8

Methods used for dose prescription (continued)

There are also other methods such as using a

dose volume histogram (DVH).

This method is particular useful for IMRT when the

evaluation of a treatment plan is based on the DVH of the target.

The method consists of assigning the prescribe dose to the

median dose in the target volume. 7.7 TREATMENT TIME AND MONITOR UNIT CALCULATIONS

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Methods used for dose prescription (continued)

An example is shown

in the DVH left: The median dose is the dose at the 50% volume level.

Since this method is not applicable in manual dose

calculations, it is not further explained in the following slides.

median dose

7.7 TREATMENT TIME AND MONITOR UNIT CALCULATIONS

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.7 Slide 10

Methods used for adding the dose at the ICRU point from multiple fields:

(1) The most simple method (usually not used):

Each field contributes to the total prescribed dose at the ICRU point using an equal number of MU (or equal treatment time).

(2) Each field contributes to the total prescribed dose at the

ICRU point with different weights. Prescribed weights for individual fields may refer to:

• the ICRU point IP (used for the isocentric techniques)
• the point of maximum dose Dmax of each field

(used for fixed SSD techniques) 7.7 TREATMENT TIME AND MONITOR UNIT CALCULATIONS

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.7 Slide 11

In the following slides two examples are shown to

calculate treatment time or monitor units when using different weights at the ICRU point.

The used method is divided into 5 steps and is based on

well known central axis formulas for the dose calculation (at the ICRU point).

Note:

This method deviates slightly from that given in the Handbook. 7.7 TREATMENT TIME AND MONITOR UNIT CALCULATIONS

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.7 Slide 12

The five steps are:

(1) Get the calibrated output of the machine at the calibration

reference point.

(2) Determine the dose at the ICRU point (IP) from each

beam, initially for an arbitrary value of 100 MU.

(3) Rescale the MUs such that the dose contributions (at IP

• r Dmax) are proportional to the pre-defined weights and

sum up the total resultant dose using the rescaled MUs.

(4) Determine the ratio between the prescribed dose and the

sum dose at IP obtained in step 3.

(5) Rescale again the MUs (from step 3) by the ratio

• btained in step 4 to get finally the required MU.

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.7 Slide 13

Step 1: Calibrated output of the machine

• For kilovoltage X ray generators and teletherapy units the
• utput is usually given in Gy/min.
• For clinical accelerators the output is given in Gy/MU.
• For superficial and orthovoltage beams and occasionally

for beams produced by teletherapy radioisotope machines, the basic beam output may also be stated as the air kerma rate in air (Gy/min) at a given distance from the source and for a given nominal collimator or applicator setting. 7.7 TREATMENT TIME AND MONITOR UNIT CALCULATIONS

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.7 Slide 14

The output for a radiotherapy machine is usually stated:

• in a water phantom
• as the dose rate for a point P

at a reference depth zref (often the depth of maximum dose zmax)

• for a nominal source to surface

distance (SSD) or source to axis distance (SAD), and

• a reference field size Aref

(often 10 × 10 cm2) on the phantom surface or the isocenter.

f = SSD zref Aref

water phantom

P

7.7 TREATMENT TIME AND MONITOR UNIT CALCULATIONS

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.7 Slide 15

The second step is performed differently depending on

whether the fixed SSD set-up or the isocentric set-up is used. fixed SSD isocentric technique 7.7 TREATMENT TIME AND MONITOR UNIT CALCULATIONS

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.7.1 Slide 1

7.7 TREATMENT TIME AND MONITOR UNIT CALCULATIONS

7.7.1 Calculations for fixed SSD set-up

ICRU point

[%]

• 150
• 140
• 130
• 120
• 100
• 70
• 50

Example of isodose distributions of a three field treatment of the prostate using fixed SSD on a 6 MV linac

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.7.1 Slide 2

Anterior field: 7.5×7.5 cm2 open field weight W = 1.0 left posterior field: 6.5×7.5 cm2 wedge field weight W = 0.8 wedge factor WF =0.53 right posterior field: 6.5×7.5 cm2 wedge field weight W = 0.8 wedge factor WF =0.53 Field parameters as obtained from the treatment planning: 7.7 TREATMENT TIME AND MONITOR UNIT CALCULATIONS

7.7.1 Calculations for fixed SSD set-up

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.7.1 Slide 3

ICRU point

[%]

• 150
• 140
• 130
• 120
• 100
• 70
• 50

Note: The prescribed weights refer to the point of maximum dose in each field: PA W = 1.0 PRPO W = 0.8 PRPO W = 0.8 7.7 TREATMENT TIME AND MONITOR UNIT CALCULATIONS

7.7.1 Calculations for fixed SSD set-up

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ICRU point is at isocenter: 152%

[%]

• 150
• 140
• 130
• 120
• 100
• 70
• 50

A normalization was firstly performed for each field individually: anterior field: Dmax refers to 100% right posterior field: Dmax refers to 80% left posterior field: Dmax refers to 80% The isodose lines are then obtained by summing up the individual %-values.

Method of normalization: 7.7 TREATMENT TIME AND MONITOR UNIT CALCULATIONS

7.7.1 Calculations for fixed SSD set-up

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.7.1 Slide 5

Step 2: For each field i, the dose at the ICRU point, Di(IP), is calculated by (using 100 MU):

i max ref

( , , . ( ) ( , , , ) ( , ) 100 100 , ) PDD z A f D IP D z A f E RDF A E WF E = ⋅ ⋅ ⋅ ⋅

where is the calibrated output of the machine is the percentage depth dose value WF is the wedge factor RDF(A,E) is the relative dose factor (see next slide)

( , , , ) PDD z A f E

max ref

. ( , , , ) D z A f E

7.7 TREATMENT TIME AND MONITOR UNIT CALCULATIONS

7.7.1 Calculations for fixed SSD set-up

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.7.1 Slide 6

The relative dose factor RDF describes the field size dependence: For a given beam energy E, the dose at the calibration point P (at depth zref) depends on the field size A. The ratio of dose to that of reference field size Aref is called the output factor, also known as total scatter factor. The IAEA Handbook is using the expression: relative dose factor (RDF): 7.7 TREATMENT TIME AND MONITOR UNIT CALCULATIONS

7.7.1 Calculations for fixed SSD set-up

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.7.1 Slide 7

P ref P ref ref

( , , , ) ( , ) ( , , , ) D z A SSD E RDF A E D z A SSD E =

RDF is defined as: Aref 7.7 TREATMENT TIME AND MONITOR UNIT CALCULATIONS

7.7.1 Calculations for fixed SSD set-up

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.7.1 Slide 8

Step 3: Rescale the MUs such that the dose contributions at Dmax are proportional to the pre-defined weights and sum up the total resultant dose using the rescaled MUs.

Σ(dose) = 148.96

39.7 152 80% =78.4 0.8 51.4 100 right post. 39.7 152 80% =78.4 0.8 51.4 100 left post. 69.5 100 100%=98.0 1.0 98.0 100 anterior dose at IP rescaled MU weighted dose at Dmax weight dose at Dmax starting MU field

7.7 TREATMENT TIME AND MONITOR UNIT CALCULATIONS

7.7.1 Calculations for fixed SSD set-up

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.7.1 Slide 9

Step 4: Determine the ratio between the prescribed dose and the sum dose at IP obtained in step 3. Prescribed dose = 200 cGy Calculated dose = 148.96 cGy

200 ratio 1.343 148.96 = =

7.7 TREATMENT TIME AND MONITOR UNIT CALCULATIONS

7.7.1 Calculations for fixed SSD set-up

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.7.1 Slide 10

Step 5: Rescale again the MUs (from step 3) by the ratio

• btained in step 4 to get finally the required MU.
• anterior field:

100 MU x 1.343 = 134 MU

• left posterior field:

152 MU x 1.343 = 205 MU

• right posterior field:

152 MU x 1.343 = 205 MU 7.7 TREATMENT TIME AND MONITOR UNIT CALCULATIONS

7.7.1 Calculations for fixed SSD set-up

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.7.2 Slide 1

7.7 TREATMENT TIME AND MONITOR UNIT CALCULATIONS

7.7.2 Calculations for isocentric set-ups

Example for an isodose distribution obtained for a 3 field prostate boost treatment with an isocentric technique

ICRU point 240%

In this example, the normalization was performed for each beam individually such that Disocenter is 100% times the beam weight. The isodose lines are then obtained by summing up the individual %-values.

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.7.2 Slide 2

Anterior field: 8×8 cm2 open field PDD = 70.9, W = 1.0 left posterior field: 7×8 cm2 wedge field PDD = 50.7, W = 0.7 wedge factor WF =0.53 right posterior field: 7×8 cm2 wedge field PDD = 50.7, W = 0.7 wedge factor WF =0.53 Field parameters as obtained from the treatment planning: 7.7 TREATMENT TIME AND MONITOR UNIT CALCULATIONS

7.7.2 Calculations for isocentric set-ups

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.7.2 Slide 3

Step 2: For each field i, the dose at the ICRU point, Di(IC), is calculated by (using 100 MU):

i max ref

. ( ) ( , , , ) ( , ) 10 ( , ) TM D IC D z A f E RDF A E R A z ISF WF = ⋅ ⋅ ⋅ ⋅ ⋅ where: is the calibrated output of the machine is the tissue-maximum-ratio at depth z WF is the wedge factor RDF(A,E) is the relative dose factor ISF is the inverse-square factor (see next slide) ( , ) TMR A z

max ref

. ( , , , ) D z A f E

7.7 TREATMENT TIME AND MONITOR UNIT CALCULATIONS

7.7.2 Calculations for isocentric set-ups

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.7.2 Slide 4

When the calibrated output factor is used in isocentric calculations, it must be corrected by the inverse- square factor ISF unless the machine is actually calibrated at the isocenter:

max ref

. ( , , , ) D z A f E

max 2

SSD z ISF SSD + ⎡ ⎤ = ⎢ ⎥ ⎣ ⎦ 7.7 TREATMENT TIME AND MONITOR UNIT CALCULATIONS

7.7.2 Calculations for isocentric set-ups

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.7.2 Slide 5

Step 3: Rescale the MUs such that the dose contributions at the IP are proportional to the pre-defined weights and sum up the total resultant dose using the rescaled MUs.

Σ(dose) = 175.44

178 70% =51.2 0.7 28.7 100 right post. 178 70% =51.2 0.7 28.7 100 left post. 100 100%=73.1 1.0 73.1 100 anterior rescaled MU weighted dose at IP weight dose at IP starting MU field

7.7 TREATMENT TIME AND MONITOR UNIT CALCULATIONS

7.7.2 Calculations for isocentric set-ups

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Step 4: Determine the ratio between the prescribed dose and the sum dose at IP obtained in step 3.

• Prescribed dose = 200 cGy
• Calculated dose = 175.44 cGy

200 ratio 1.140 175.44 = =

7.7 TREATMENT TIME AND MONITOR UNIT CALCULATIONS

7.7.2 Calculations for isocentric set-ups

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.7.2 Slide 7

Step 5: Rescale again the MUs (from step 3) by the ratio

• btained in step 4 to get finally the required MU.
• anterior field:

100 MU x 1.14 = 114 MU

• left posterior field:

178 MU x 1.14 = 203 MU

• right posterior field:

178 MU x 1.14 = 203 MU 7.7 TREATMENT TIME AND MONITOR UNIT CALCULATIONS

7.7.2 Calculations for isocentric set-ups

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.7.3 Slide 1

7.7 TREATMENT TIME AND MONITOR UNIT CALCULATIONS

7.7.3 Normalization of dose distributions

Important: Dose distributions can be normalized in different ways: normalized to maximum dose normalized such that 100% = 100cGy

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.7.3 Slide 2

Frequently the dose distribution is normalized to the

maximum dose.

The ICRU recommends normalization of the dose

distribution to 100% at the prescription point.

As a consequence, values of the dose distribution larger

than 100% will be obtained if the prescription point is not located at the point of maximum dose.

If the isodose values generated by the TPS itself are used

for the monitor calculations, the method of normalization used in the TPS must be taken into account. 7.7 TREATMENT TIME AND MONITOR UNIT CALCULATIONS

7.7.3 Normalization of dose distributions

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.7.4 Slide 1

7.7 TREATMENT TIME AND MONITOR UNIT CALCULATIONS

7.7.4 Inclusion of output parameters in dose distribution

Modern treatment planning systems give the user the

ability to take into account several dosimetric parameters in the dose distribution affecting the beam output.

For example, the isodose values in a dose distribution

may already include:

• inverse square law factors for extended distance treatments,
• effects on dose outputs from blocks in the field,
• tray and wedge factors.

If the isodose values generated by the TPS are used for

the monitor calculations, it is of utmost importance to know exactly what the isodose lines mean.

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Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 7.7.5 Slide 1

7.7 TREATMENT TIME AND MONITOR UNIT CALCULATIONS

7.7.5 Orthovoltage and cobalt-60 units

Treatment time calculations for orthovoltage units and

cobalt-60 teletherapy units are carried out similarly to the above examples except that machine outputs are stated in cGy/min and the treatment timer setting in minutes replaces the monitor setting in MU.

A correction for shutter error should be included in the

time set.